Lactoferrin (Lf), the principal and most avid Fe-binding protein in milk and other epithelial secretions and the major constituent of the specific granules of neutrophils, has been implicated in a wide variety of functions. Its chief role appears to be defense against infectious agents, through strong chelation of the Fe required for microbial growth, but functions as diverse as Fe absorption in the gut, feedback suppression of myelopoiesis and potent stimulation of macrophage and NK toxicity, among many others, have been attributed to the molecule. Some of these functions appear independent of Fe binding, and some relate to the strong but unexplained binding affinity of Lf for nucleic acids. Defects in Lf expression are observed in some cancers, and may contribute to the pathogenesis or clinical manifestations of the disease. By virtue of its profound affinity for Fe and multitude of related and unrelated effects, Lf offers interesting potential as a therapeutic in infectious diseases and neoplasms or as an immunomodulator. However, many of these proposed functions of Lf are controversial and virtually all are unproven at the physiological level. We have made the unique observation that there are multiple forms of Lf. At least one isoform binds no Fe and instead, expresses a potent ribonuclease activity. These isoforms have similar, if not identical, primary structures and are not interconvertible. It is our long-term objective to determine how the varied and powerful functions of Lf can be exploited therapeutically. Our working hypothesis is that the existence of multiple forms of Lf explains the broad and often inconsistent functions proposed for Lf. In order to test this hypothesis and ultimately to realize the therapeutic potential for the isoforms, we must gain an understanding of Lf pathophysiology and determine the precise in vivo role for each isoform. We also have the opportunity to determine how isoforms with such similar structures could express such disparate and nonoverlapping activities. This will be accomplished by: 1. determining the structural differences that account for the different activities of the isoforms; 2. determining the significance of the avid DNA binding activity of Lf isoforms; 3. cloning and expressing the isoforms; 4. examining structure-function relationships in the isoforms using site-directed mutagenesis; 5. analyzing the regulatory sequences that direct the unique tissue distribution of Lf; 6. constructing transgenic lines of mice in which the Lf gene has been disrupted and testing their phenotype and responsiveness to infection and neoplasia.